The present invention relates to a liquid-encapsulated Czochralski method for single crystal growing of a compound semiconductor or, more particularly, to a liquid-encapsulated Czochralski (LEC) method for growing a single crystal of a compound semiconductor which is substantially free from dislocations and undesirable crystalline lattice defects of other types.
As is well known, single crystals of III-V Group compound semiconductors such as gallium arsenide GaAs are widely used as a material of high-performance electronic components such as high-speed discrete devices, high-speed integrated circuits and the like by virtue of the remarkably large electron mobility therein. A problem in such high-performance devices and integrated circuits is that the semiconductor substrate thereof must be prepared from a dislocation-free single crystal of which the electric properties are uniform to an extremely small microscopic dimension. In this regard, extensive investigations have been undertaken by the LEC method but no successful results have hitherto been reported that a dislocation-free undoped single crystal can be obtained. The best of the methods known at present and the most widely utilized method at present is to admix the melt of gallium arsenide with indium in a relatively minor amount so that the single crystal grown from the melt is imparted with increased strengths along with consequent prevention of occurrence of dislocations in the bulk as is reported in Journal of Crystal Growth, volume 61 (1983), pages 417-424.
In the above mentioned method, occurrence of dislocations can be prevented by increasing the amount of indium added to the melt of gallium arsenide. According to this technology, a single crystal of gallium arsenide having a large diameter but substantially free from dislocations can be grown only when the concentration of indium is considerably high in the melt of gallium arsenide.
When the additive added to the melt has an equilibrium segregation coefficient K.sub.o smaller than 1, as is the case with indium in gallium arsenide, however, the concentration of the additive in the melt left in the crucible is steadily increased as the single crystal growing proceeds and constitutional supercooling at the interface of the growing front of the single crystal and the melt may eventually cause cellular growth. The above mentioned scientific journal article reports that, in such a case, a large number of dislocations are produced in the single crystal or, in some extreme cases, no single crystal can be obtained but what is grown up is a polycrystalline material.
This problem can of course be solved by performing the liquid-encapsulated Czochralski crystal growing at an extremely low velocity of pulling up. For example, the velocity of pulling up should be 1.0 mm/hour or lower in a high solidification range where the solidification is defined as the ratio of the weight of the single crystal under proceeding growing to the total amount of the melt formed in the crucible at the start of crystal growing including the additives. Such a method is of course not practical due to the extremely low productivity or high costs.
On the other hand, it is also desirable that the process of crystal growing is carried out until the extent of solidification is as high as possible while the undesirable phenomenon of cellular growth sometimes takes place in the latest stage of such a crystal growing process.